This type of a conventional charging control unit is for controlling charging to efficiently charge with fluctuating power from a solar cell.
FIG. 13 is a block diagram illustrating an exemplary configuration of essential parts of a conventional charging control unit. FIG. 14 is a diagram illustrating an output voltage—output current characteristic of a solar cell with photoirradiation intensity as a parameter.
In FIG. 13, as a conventional charging control unit 100, a charging control circuit 102 which enhances charging efficiency of a power instrument 103 and is suitable for a protection feature required in accordance with the type of the power instrument 103 when charging the power instrument 103, such as a secondary batter or a super capacitor, with power generated by a solar cell 101 receiving light, is used.
Further, power generated by a natural energy charger represented by the solar cell 101 greatly fluctuates in accordance with natural energy. Thus, an MPPT control feature (MPPT: Maximum Power Point Tracking) compliant with maximum power, which is needed to draw out generated power that fluctuates to the maximum degree, is required as a specification of such a charging control unit. The amount of power generated by the solar cell 101 increases in accordance with irradiation intensity of sun light. The relationship between the irradiation intensity and a power curve is as illustrated in FIG. 14.
The charging control unit 102 is a device prepared for transmitting power of the solar cell 101 to the power instrument 103 (secondary battery, super capacitor or the like). Thus, the driving power therefor should be sufficiently met with only power generated from the solar cell 101. When this is not possible, a reverse flow of power from the power instrument 103 to the charging control circuit 102 occurs while controlling charging, resulting in loss of power. For this reason, it is necessary to determine whether power generated by the solar cell 101 is equal to or greater than the driving power for the charging control circuit 102. A UVLO (Under Voltage Lock Out) circuit 104 for monitoring an output voltage of a solar cell to determine whether the voltage has risen to a prescribed voltage is employed as a representative activation determining circuit. The conventional charging control unit 100 is constituted with such a charging control circuit 102 and a UVLO circuit 104.
The UVLO circuit 104 is an activation determining circuit that is widely employed in common power source ICs. The UVLO circuit 104 is activated when a primary power source voltage (in this case, solar cell output voltage) is equal to or greater than a prescribed voltage condition. In many cases, the UVLO circuit 104 has a hysteresis feature for the determination voltage in order to prevent successive malfunctions, such as activate→stop→activate, due to UVLO observed voltages fluctuating from a rush current to a capacitor that can occur when the charging control circuit 102 is activated or another rush current at the time of activating a circuit. The UVLO circuit 104 is commonly used and is also disclosed in Patent Literature 1.